The present invention relates to a far field monitor apparatus and, more particularly, to double-side ILS (Instrument Landing System) approach far field monitor apparatus using the integral monitor circuit of an opposing-side ILS.
Generally, an airport where aircraft departs/arrives uses an ILS which helps aircraft to land using radio waves. As one of apparatuses constructing the ILS, a localizer for guiding to a landing aircraft a horizontal shift from the runway center is installed.
The localizer radiates, from the front side of a landing aircraft, carrier and sideband patterns having components modulated by 90 Hz and 150Hz and different directivities. The aircraft receives the radio waves from the localizer and derives parameters including the DDM (Difference of Depth of Modulation) between the two radio waves, thereby detecting the horizontal shift from the runway center. Hence, it must be monitored at a predetermined accuracy whether the radio waves are radiated from the localizer.
Conventionally, a far field monitor apparatus has been proposed as an apparatus for monitoring radio waves radiated from a localizer and, more particularly, an apparatus for monitoring a radiation characteristic in the far field of a localizer. FIG. 8 shows a conventional far field monitor apparatus.
Referring to FIG. 8, a far field monitor antenna 100 comprises three receiving antennas 101a to 101c and receives radio waves radiated from a localizer. Receiving signals from the receiving antennas 101a to 101c are combined by a combiner 102, divided into three signals by a divider 103, and output to ILS receivers 104a to 104c. 
The ILS receivers detect monitor signals from the receiving signals and output the monitor signals to detectors 105a to 105c each incorporating a timer circuit. When the monitor signal level exceeds a predetermined value, and its duration exceeds a predetermined value, the detectors 105a to 105c output alarm signals to a majority determination circuit 106. The majority determination circuit 106 determines the majority of received alarm signals and outputs a control signal 107 for transferring or shutting down the transmitter.
The operation of the conventional far field monitor apparatus will be described next.
The receiving antennas 101a to 101c are directivity antennas represented by log-periodic antennas and sequentially installed on an extended runway center line 108 at an interval of 30 m. ILS localizer signals received by the receiving antennas 101a to 101c are vector-combined by the combiner 102, divided into three signals by the divider 103, and supplied to the ILS receivers 104a to 104c, i.e., onboard receivers, respectively.
The ILS receivers 104a to 104c detect DDMs and flag currents (FLAGs) from the receiving signals and output them to the detectors 105a to 105c, respectively. When the values of DDMs or FLAGs as monitor signals exceed a predetermined value, the internal timer circuits of the detectors 105a to 105c start measuring duration.
If the duration exceeds a predetermined time within the range of 30 to 240 sec, the detectors 105a to 105c determine xe2x80x9calarmxe2x80x9d and output alarm signals to the majority determination circuit 106. The actual duration is determined depending on the operation situation of each airport. When it is determined using a predetermined alarm determination scheme, e.g., ⅔ majority determination scheme that two or more detectors have output the alarm signals for one monitor item, the majority determination circuit 106 outputs the control signal 107 for transferring or shutting down the transmitter.
FIGS. 9A and 9B show an installation example of the conventional far field monitor apparatus.
To monitor, equivalently as in the far field, the radiation characteristic of an array antenna having a plurality of antenna elements and an aperture 57 as large as 21 to 38 m, the monitoring must be done at a position separated by about 1,060 m that is calculated by
r xe2x89xa72xc3x97(AP)2/xcexc
where r is the minimum distance (m), AP is the aperture (=38 m), and xcex is the wavelength (m) (=300/110 (MHz)=2.727 (m)).
Normally, the far field monitor antenna 100 is installed at one of a rear position 109 of the middle marker on the extended runway center line 108, an intermediate position 110 between the middle marker and the inner marker, and an intermediate position 111 between the inner marker and the opposing-side localizer. The antenna height is set within a range not exceeding a 2% approach surface 58 defined as an obstacle clearance such that an outlook over an operation-side ILS localizer 60 can be ensured.
Reference numeral 36 denotes a runway; 37, a runway center line; 39, a threshold; 40, a stop end; 55, a localizer course; 56, a course width; 60, an operation-side ILS localizer; 61, an opposing-side localizer; 62, an inner marker; and 63, a middle marker.
However, such a conventional far field monitor apparatus suffers the following problems.
As the first problem, the indicator of the far field monitor is unstable. This is because the far field monitor is arranged on the localizer course and therefore readily affected by secondary reflected waves from approaching or departing aircraft or aircraft moving on the runway.
As the second problem, an error in DDM as one of monitor signals is not always detected. This is because two or three antennas are installed along the flight course, and receiving signals are averaged in practice, although a number of antennas are preferably used to monitor the average course in the far field. For this reason, when the direct waves and reflected waves are combined in opposite phases, or the direct waves are shielded by an obstacle, the apparent DDM is determined to fall within the predetermined value range, and no error is detected, although the receiving signals are at noise level.
As the third problem, to reliably receive and monitor weak radio waves at a far position close to the ground surface, the receiving antenna becomes high to increase the risk for aircraft. The reason is as follows. When the antenna is installed near, e.g., the middle marker, the distance to the receiving point is as long as about 5 km. For this reason, the receiving field strength decreases due to the influence of a decrease in angle of incidence due to the curvature of earth, i.e., an increase in vertical plane pattern loss. Hence, to ensure the visibility over the transmission point and ensure the receiving field strength, the height of receiving antenna must be increased within the range not exceeding the approach surface.
As the fourth problem, the apparatus has a large scale, and the installation cost is high. To install the far field monitor apparatus including three receiving antennas at an interval of 30 m at the rear position of the middle marker, the intermediate position between the middle marker and the inner marker, or the intermediate position between the middle marker and the inner marker, a wide area must be ensured, and large-scale construction is required.
It is an object of the present invention to provide a stable far field monitor apparatus free from detection error even when it is affected by secondary reflected waves from aircraft and the like.
It is another object of the present invention to provide an inexpensive and simple far field monitor apparatus having a small height.
In order to achieve the above objects, according to the present invention, there is provided a far field monitor apparatus having first and second ILS (Instrument Landing System) localizers opposing via a runway in a longitudinal direction, each of the first and second ILS localizers comprising receiving antenna means formed from a plurality of antenna elements symmetrically arranged in a direction perpendicular to the longitudinal direction of the runway, the receiving antenna means of the first and second ILS localizers opposing each other, a plurality of coupling means, arranged in units of antenna elements, for picking up some of signals obtained by the antenna elements in a predetermined amplitude and phase, combine means for combining output signals from the coupling means, receiving means for receiving a combined signal output from the combine means, and first detection means for detecting, on the basis of a receiving signal output from the receiving means, a predetermined monitor parameter representing a radiation state of a radio wave radiated from the opposing-side ILS localizer.